DESIGN OF ELECTRICAL MACHINES (DEM)–Nov / Dec 2011 Question Paper
Anna University
B.E./B.Tech. DEGREE EXAMINATION
NOVEMBER/DECEMBER 2011.
Sixth Semester
Electrical and Electronics Engineering
EE 2355 — DESIGN OF ELECTRICAL MACHINES NOVEMBER/DECEMBER 2011.
(Regulation 2008)
Time : Three hours
Maximum : 100 marks
Answer ALL questions.
PART A — (10 × 2 = 20 marks)
1. What are the major considerations in Electrical Machine Design?
2. Write down the classification of magnetic materials.
3. Show how the specific magnetic and electric loadings are interdependent.
4. Mention any two guiding factors for the choice of number of poles.
5. Define the term: ‘Voltage Regulation’.
6. What are the methods by which heat dissipation occurs in a transformer?
7. Write down the equation for output coefficient in an Induction Motor.
8. Define : Stator Slot Pitch.
9. What are the factors that influence the choice of specific magnetic loading in a synchronous machine?
10. Define Short Circuit Ratio of a synchronous machine.
PART B — (5 × 16 = 80 marks)
11. (a) What are the main groups of Electrical conducting materials?
Describe the properties and applications of those materials. (16)
Or
(b) Describe any two methods used for determination of motor rating for variable load drives with suitable diagrams. (16)
12. (a) Explain the various steps involved in the design of Armature winding of D.C. machine. (16)
Or
(b) | A design is required for a 50 kW, 4 pole, 600 rpm, d.c. shunt generator, the full load terminal voltage being 220 V. If the maximum gap density is 0.83 Wb/m2 and the armature ampere conductors per metre are 30,000, calculate suitable dimensions of armature core to give a square pole face. | |
Assume that the full load armature voltage drop is 3 percent of the rated terminal voltage and that the field current is 1 percent of rated full load current. Ratio of pole arc to pole pitch is 0.67. (16) | ||
13. | (a) | Discuss about temperature rise and methods of cooling in transformers. |
Or
(b) | A 6600 V, 60 Hz single phase transformer has a core of sheet steel. The net iron cross-sectional area is 22.6 × 10–3 m2, the mean length | ||
is 2.23 m, and there are four lap joints. Each lap joints takes ¼ times as much reactive mmf as is required per metre of core. If Bm = 1.1 Wb/m2, determine
| |||
(i) The number of turns on the 6600 V winding and | |||
(ii) The no load current. Assume an amplitude factor of 1.52 and that for given flux density, mmf per metre = 232 A/m; specific loss = 1.76 W/kg. Specific gravity of plates = 7.5. (16)
| |||
14. | (a) | Describe the steps involved in the design of magnetising current for an induction motor from design data. | (16) |
Or
(b) Estimate the stator core dimensions and the total number of stator conductors for a 3 Φ , 100 kW, 3300 V, 50 Hz, 12 pole star connected slip ring Induction motor. Assume : average gap density = 0.4 Wb/m2, conductors per metre = 25,000 A/m, efficiency = 0.9, power factor = 0.9 and winding factor =0.96.
Choose main dimension to give best power factor. (16)
15. (a) Explain the step by step procedure for the design of field winding ofSynchronous machine. (16)
Or
(b) | A 1000 KVA, 3300 V, 50 Hz, 300 rpm, 3-phase alternator has 180 slots with 5 conductors per slot. Single layer winding with full pitch | |
coils is used. The winding is star connected with one circuit per phase. Determine the specific electric and specific magnetic loadings, if the stator bore is 2.0 m and the core length is 0.4 m. The machine | ||
has 60° phase spread. | (16) |
Anna University
B.E./B.Tech. DEGREE EXAMINATION, APRIL/MAY 2011
Sixth Semester
Electrical and Electronics Engineering
EE 2355 — DESIGN OF ELECTRICAL MACHINES APRIL/MAY 2011
(Regulation 2008)
Time : Three hours
Maximum : 100 marks
Answer ALL questions
PART A — (10 × 2 = 20 marks)
1. What is specific Electric Loading?
2. How materials are classified according to their degree of magnetism?
3. Name any two methods to reduce armature reaction.
4. What is slot loading?
5. Give the relationship between emf per turn and kVA rating in a transformer.
6. What are the factors affecting the choice of flux density of core in a transformer?
7. How crawling can be prevented by design in an Induction motor?
8. Define dispersion coefficient of an Induction Motor.
9. What is run away speed of Synchronous Machine?
10. Give the need for damper winding in Synchronous Machine.
PART B — (5 × 16 = 80 marks)
11. (a) Discuss about various duties and ratings of Rotating Machines and give their respective temperature — time curves. (16)
Or
(b) A field coil has a heat dissipating surface of 0.15 m2 and length of mean turn 1 m. It dissipates loss of 150 W, the emissivity being 34 W/m2_°C. Estimate the final steady temperature rise of the coil and its time constant if the cross section of the coil is 100*50 mm2. Specific heat of copper is 390 J/kg° C. The space factor is 0.56. Copper weighs 8900 kg/m3. (16)
12. (a) (i) Explain the effects of choice of number of poles in a DC Machine on (1) Frequency of flux reversal (2) Weight of iron (3) Weight of copper and (4) Length of commutator. (8)
(ii) A 5 kw, 250 V, 4 pole, 1500 rpm DC Shunt Generator is designed to have a square pole face. The specific magnetic loading and specific electric loadings are 0.42 Wb/m2 and 15000 AC/m respectively. Find the main dimensions of the machine. Assume full load efficiency = 0.87 and pole arc to pole pitch ratio is 0.66. (8) Or
(b) (i) Discuss various methods to determine mmf required for teeth of anElectric Machine. (8)
(ii) Determine the apparent flux density in teeth of a DC machine if the real flux density in teeth is 2.15 Wb/m2, slot pitch is 28 mm, slot width is 10 mm, gross core length is 0.35 m, no. of ventilating ducts is 4 each 10 mm wide. Magnetizing force corresponding to flux density of 2.15 Wb/m2 is 55000 AT/m and iron stacking factor is 0.9. (8)
13. (a) (i) Derive the output equation of a three phase transformer. (8)
(ii) The ratio of flux to full load mmf in a 400 kVA, 50 Hz single phase core type transformer is 2.4*10–6. Calculate the net iron area and the window area of the transformer if the maximum flux density in the core is 1.3 Wb/m2, Current density is 2.7 A/mm2 and window space factor is 0.26. Also calculate the full load mmf. (8)
Or
(b) A 250 kVA, 6600/400V three phase core type transformer has a total loss of 4800 W at full load. The transformer tank is 1.25 m in height and 1 m × 0.5 m in plan. Design a suitable scheme for tubes if the average temperature rise is to be limited to 35° C. The diameter of each tube is 50 mm and are spaced 75 mm from each other. The average height of tubes is 1.05 mm. Specific heat dissipation due to radiation and convection is respectively 6 and 6.5 W/bm2_°C. Assume that convection is improved by 35% due to the provision of tubes. (16)
14. (a) Determine the main dimensions, number of radial ventilating ducts, number of stator slots and turns per phase of a 3.7 kW, three phase, 400 V, 4 Pole, 50 Hz squirrel cage Induction Motor to be started by a Star-Delta starter. Given that the average flux density in the air gap = 0.45 Wb/m2; Ampere Conductor per meter of armature periphery = 23000, full load efficiency = 0.85, full load power factor = 0.84 and kw = 0.955. Take L/ τ = 1.5. (16)
Or
(b) (i) Discuss the factors to be considered in estimating the length of air gap of an Induction Motor. (8)
(ii) Discuss the step by step procedure to design the rotor of a squirrel cage Induction Motor. (8)
15. (a) Define Short Circuit Ratio. Explain how it is determined for an alternator. Also discuss its effects on the performance of alternator. (16)
Or
(b) (i) Derive the output equation of an AC machine. (8)
(ii) Determine the main dimensions of a 100 kVA, 50 Hz, three phase 375 rpm alternator. The average air gap flux density is 0.55 Wb/m2 and ampere conductors per metre is 28000. Given that τ / L must be between 1 to 5. The maximum permissible peripheral speed is 50 m/sec. The runaway speed is 1.8 times synchronous speed. (8)